drm/i915/perf: Validate OA sseu config outside switch

Once OA supports media engine class:instance, the engine can only be
validated outside the switch since class and instance parameters are
separate entities. Since OA sseu config depends on engine
class:instance, validate OA sseu config outside the switch.

v2: (Ashutosh)
- Clarify commit message
- Use drm_dbg instead of DRM_DEBUG
- Reorder stack variables

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230323225901.3743681-4-umesh.nerlige.ramappa@intel.com